I've been a huge Namco fan all my life and Namco System 2 has some of my favorite games of all time. So I'll chime in on a technical and practical level with my thoughts on the possibility of a multi for NS2:
Namco System 2 uses different video boards for different types of games. There's a driving board for Final Lap/Suzuka/Four Trax, a unique board for Steel Gunner, a unique board for Lucky and Wild and a unique board for Metal Hawk. (There are two revisions of the CPU board, the custom 65 version and the custom 68 version. All CPU boards are interchangable).
CPU boards are (relatively) cheap. You can find parted out Final Laps, Suzukas and Four Trax machines all the time on eBay. If you want to play these games, just go hunt down the boards. For the purposes of the multi WE DO NOT CARE ABOUT THESE GAMES.
If we rule out all of the games that do NOT run on the standard video board we are left with these games:
(And let's be honest here, because they are driving or shooting games most people won't have a way to play them anyway)
From here, let's rule out the baseball games that most people won't care about and games we cannot play due to having a unique cabinet such as Golly Ghost (a game that is mostly electromechanical). That leaves us with:
These are the games we actually care to play on Namco System 2. 13 games, and since Assault Plus is just an upgraded version of Assault we can say that Namco System 2 has 12 unique games that we care about. These game boards hold value by themselves that wouldn't be used as donors to convert to other games, such as the baseball games.
Is it worth it to have a multi for just these games? I dunno. I suppose Namco System 1 has roughly the same amount of playable games. So this question is more for Darksoft.
From a technical perspective I think a multi is worth it if Darksoft can completely reproduce the video PCB.
This is the Video PCB
The PCB is relatively simple, it consists of mostly TTL logic, some RAM, and two Namco Customs. The customs are the hurdle here. There's the 102 that controls ROZ ROMs memory access control and the 146 which controls the OBJ ROMs (line buf steering according to MAME)
If Darksoft can successfully reverse engineer these two customs and reproduce the entire video board then I think a Namco System 2 multi would be a great product. Imagine buying out all the useless Final Laps, Suzuka and Four Trax PCBs and making something useful out of them. The downside here is that the multi would probably be very expensive to produce. You still have to come up with multi ROMs that go on the CPU board as well. The upside is not having to hunt down and sacrifice the extremely rare and expensive original video PCBs.
If Darksoft goes the other route and makes a multi that requires hunting down the original video PCB, it will be more cost effective on his end but then good luck finding a reasonably priced Namco System 2 board as a donor.
Anyway that's my two cents. I remember when Valkyrie boards went for $250 all day every day on Yahoo Japan and I thought "that's way too expensive". Now people want $1000+ for them.